1. Field of the Invention
The present invention relates to a radio frequency device having a metal shielding cap structure.
2. Description of the Prior Art
In recent years, information communications have played an extremely important part, and demands for mobile communication systems have rapidly been increasing. Under such circumstances, for mobile telephones and cordless telephones, because of demands for system miniaturization, it is becoming extremely important to miniaturize devices constituting a radio frequency circuit block.
Conventionally, as a radio frequency device which is one of the devices in a radio frequency circuit block of a portable telephone employing the PDC (personal digital cellular) system, in particular an amplifying device as a radio frequency amplifier for transmission, a gallium arsenide field effect transistor is used because of its excellent radio frequency characteristics.
FIG. 7 shows an example of the cross-sectional structure of a prior art radio frequency device (for example, see Japanese Laid-Open Patent Application No. H11-220226, pp. 3–5, FIGS. 1 and 7). In this radio frequency device, for example, a gallium arsenide field effect transistor 3 is mounted in a concave portion on the reverse surface of dielectric multilayer circuit board 2 being, for example, rectangular, and the transistor 3 is encapsulated in a resin 9. Radio frequency circuit parts 5 mounted on land patterns 10 on the surface of the dielectric multilayer circuit board 2 (the upside in FIG. 7), and a strip line 6 constitute a radio frequency matching circuit of the transistor 3. The radio frequency circuit parts 5 on the surface of the dielectric multilayer circuit board 2 (the upside in FIG. 7), and a microstrip line 7 constitute an electric circuit of the transistor 3. These circuits are interconnected by a via hole 8 in an inner layer formed by an electric conductor such as a conductive paste being filled in a minute hole for a via hole, and this forms a radio frequency microwave circuit.
The radio frequency circuit parts 5 and the strip line 6 on the surface of the dielectric multilayer circuit board 2 are shielded by a metal shielding cap 1. The metal shielding cap 1 is electrically joined to a ground pattern 4 present at an end of the top surface or at the side of the dielectric multilayer circuit board 2 by soldering, welding or the like. The ground pattern 4 is also present on the reverse surface of the dielectric multilayer circuit board.
FIG. 8 shows distances required of the design because of positional displacements and dimensional deviations of the metal shielding cap 1, the dielectric multilayer circuit board 2, the radio frequency circuit parts 5 and the land pattern 10 in the prior art radio frequency device.
Here, a structure is shown such that the side walls of the metal shielding cap 1 are placed on the ends of the top surface of the dielectric multilayer circuit board 2. As the dielectric material of the dielectric multilayer circuit board 2, in view of heat radiation, alumina that is excellent in thermal conductivity is frequently used. To form the land pattern 10 where the radio frequency circuit parts 5 are mounted and the strip line 6 on the surface of the dielectric multilayer circuit board 2, the following procedure is taken: A thick film conductor with copper as the main constituent is screen-printed on green sheets formed of the dielectric material, these are laid one on another and crimped, and the dielectric and the conductor are baked, whereby the above-described structure is completed.
However, in the steps of printing, crimping and baking, the positions of the land pattern 10 for mounting the radio frequency circuit parts 5 and the strip line 6 are displaced by a maximum of 0.100 mm (the dimension D in FIG. 8) with respect to the design CAD drawing dimension. Moreover, the following displacements and deviations are present: an outer dimension deviation of a maximum of 0.100 mm (the dimension A in FIG. 8) caused when the dielectric multilayer circuit board 2 after the baking is divided into pieces; a mounting position displacement of the radio frequency circuit parts 5 of a maximum of 0.150 mm (the dimension E in FIG. 8); an outer dimension deviation of the metal shielding cap 1 of a maximum of 0.050 mm (the dimension B in FIG. 8); and a positional displacement of a maximum of 0.150 mm (the dimension C in FIG. 8) caused when the metal shielding cap 1 is soldered or welded to the dielectric multilayer circuit board 2.
For this reason, in the conventional metal shielding cap structure, from the relationship between the mounting position accuracies of the radio frequency circuit parts 5 and the metal shielding cap 1 and the dimensional accuracies of the dielectric multilayer circuit board 2 and the metal shielding cap 1, a predetermined distance 0.550 mm (the dimension F in FIG. 8) is required to prevent the radio frequency circuit parts 5 and the metal shielding cap 1 from being in contact with each other, and this has been an obstacle to miniaturization.
Likewise, in the prior art metal shielding cap structure, from the relationship among the positional accuracy of the strip line 6, the mounting position accuracy of the metal shielding cap 1 and the dimensional accuracies of the dielectric multilayer circuit board 2 and the metal shielding cap 1, a predetermined distance 0.450 mm is required to prevent the strip line 6 and the metal shielding cap 1 from being in contact with each other, and this has been an obstacle to miniaturization.
Further, the presence of these positional displacements and dimensional deviations means that the distance between the radio frequency circuit part 5 and the metal shielding cap 1 cannot be maintained constant, and the above-mentioned displacements and deviations affect the impedances of the radio frequency circuit parts 5. This affects the radio frequency matching of the transistor, and degrades the characteristics of the radio frequency device.
Moreover, in a structure where the side walls of the metal shielding cap 1 and the ground patterns on the side surfaces of the dielectric multilayer circuit board 2 are soldered or welded to thereby join the metal shielding cap 1 and the dielectric multilayer circuit board 2 together, the following problem arises: When there is a deviation between the dimensions of the metal shielding cap 1 and the outer dimensions of the dielectric multilayer circuit board 2, the metal shielding cap 1 does not fit on the dielectric multilayer circuit board 2, or a float (gap) is caused at the junction between the metal shielding cap 1 and the dielectric multilayer circuit board 2 to cause a contact failure.